Endocrinology and hormones Dr. Ibtesam G. Auda M.Sc. , Ph.D

The endocrine system consists of several glands, all in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone.

Hormones are chemical messengers that are secreted (released) from glands into the blood and affect cells in another part of the body.

Hormones only work on certain cells, called target cells. The target cells have special receptors that “recognize” the hormones and allow them to influence that cell. Two groups of receptors: placed on the surface of membrane – peptide and protein hormones, prostaglandins; placed inside the cells (cytoplasm, nucleus) – steroid and thyroid hormones

Some endocrine glands and what they regulate

Types of Hormones. Endocrine glands produce 2 major classes of hormones (and several other types as well): 1. Protein hormones: These comprise amino acids, those that are only several amino acids in length are called peptide hormones, whereas larger ones are called polypeptide hormones. They include: Insulin: Made in the pancreas, it increases the entry of glucose into the cells, and regulates fat storage. Glucagons: Made in the pancreas, are responsible for increasing the conversion of stored fats to blood glucose. Leptin: Produced by the fat cells, it informs the brain how much fat is contained in the body.

2. Steroid Hormones a) Corticosteroides: . These are derived from cholesterol from the diet and exert their effects in two ways: i) They bind directly to membrane receptors. ii) As they are fat soluble they pass through cell membranes where they attach to receptors in the cytoplasm. Here they determine gene expression. There are several types of steroid hormones: a) Corticosteroides: . Glucocorticoids (principally cortisol) are released by the adrenal glands in response to stress. They increase the breakdown of fats and proteins into glucose to trigger escape or defense ("fight or flight"). Mineralocorticoids (e.g. aldosterone) are also produced by the adrenal glands and reduce salt secretion in the kidneys.

b) Sex Steroids These are released mainly by the ovaries and testes but also by the adrenal glands. They comprise: Androgens: Testosterone is produced in large amounts in males and has masculinising and defeminising effects; maintaining male secondary sexual characteristics and promoting courtship, aggressive and sexual behaviours. Estrogens: Estradiol is produced in large amounts in females and has feminising effects, promoting female secondary sexual characteristics, water retention, calcium metabolism, sexual behaviour and maternal behaviours. Progesterone prepares the uterus for the implantation of a fertilised ovum and regulates the stages of pregnancy.

Negative feedback means that when enough hormone is in the body, the body stops producing the hormone until it is needed again. most hormone systems are Negative feedback systems Increased glucose is detected by receptors that notify the brain. It sends a message to the pancreas to produce insulin. You eat. Glucose (sugar) in the blood increases. Insulin tells muscle and liver to take up glucose from the bloodstream and use it for energy or store it for later. Brain reduces appetite. Pancreas stops making insulin. Blood glucose level drops as it is removed by the cells.

A few hormone systems are positive feedback systems: The pituitary gland sends a signal by way of the hormone oxytocin to the uterus causing contractions. The pressure of the fetus on the cervix sends a signal back to the brain which then stimulates the release of more oxytocin. This causes more contractions. The fetus pushes harder on the cervix. More oxytocin is released. The system continues until birth occurs

hypothalamus  is a portion of the brain . One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland. It synthesizes and secretes certain neurohormones, called releasing hormones or hypothalamic hormones, and these in turn stimulate or inhibit the secretion of pituitary hormones. 

Anterior pituitary hypothalamus effect on Secreted hormone Effect Thyrotropin-releasing hormone (Prolactin-releasing hormone) Stimulate thyroid-stimulating hormone (TSH) release from anterior pituitary (primarily) Stimulate prolactin release from anterior pituitary Corticotropin-releasing hormone Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary Dopamine (Prolactin-inhibiting hormone) Inhibit prolactin release from anterior pituitary Growth-hormone-releasing hormone Stimulate growth-hormone (GH) release from anterior pituitary Gonadotropin-releasing hormone Stimulate follicle-stimulating hormone (FSH) release from anterior pituitary Stimulate luteinizing hormone (LH) release from anterior pituitary Somatostatin] (growth-hormone-inhibiting hormone) Inhibit growth-hormone (GH) release from anterior pituitary Inhibit (moderately) thyroid-stimulating hormone (TSH) release from anterior pituitary

Posterior pituitary Effect Secreted hormone Uterine contraction Lactation (letdown reflex) Oxytocin Increase in the permeability to water of the cells of distal tubule and collecting duct in the kidney and thus allows water reabsorption and excretion of concentrated urine Vasopressin (antidiuretic hormone هذه الهرمونات تفرز من الهايبوثلامس لكن تصب في الجزء الخلفي للنخامية

Role of the Pituitary The pituitary is the “master gland” that signals other glands to produce their hormones when needed. The anterior lobe of the pituitary receives signals from the hypothalamus, and responds by sending out the appropriate hormone to other endocrine glands. The posterior pituitary receives oxytocin or antidiuretic hormone (ADH) from the hypothalamus, relays them to the body as necessary.

Pituitary Hormones Pituitary Hormone Functions Follicle-stimulating hormone Stimulates egg maturation in the ovary and release of sex hormones. Lutenizing hormone Stimulates maturation of egg and of the corpus luteum surrounding the egg, which affects female sex hormones and the menstrual cycle. Thyroid-stimulating hormone Stimulates the thyroid to release thyroxine. Adrenocorticotropic hormone Causes the adrenal gland to release cortisol. Melanocyte-stimulating hormone Stimulates synthesis of skin pigments. Growth hormone Stimulates growth during infancy and puberty. Antidiuretic hormone Signals the kidney to conserve more water. Oxytocin Affects childbirth, lactation, and some behaviors.

Endocrine Hormones Gland Hormones Functions Thyroid Thyroxine Regulates metabolism Calcitonin Inhibits release of calcium from the bones Parathyroids Parathyroid hormone Stimulates the release of calcium from the bones. Islet cells (in the pancreas) Insulin Decreases blood sugar by promoting uptake of glucose by cells. Glucagon Increases blood sugar by stimulating breakdown of glycogen in the liver. Testes Testosterone Regulates sperm cell production and secondary sex characteristics. Ovaries Estrogen Stimulates egg maturation, controls secondary sex characteristics. Progesterone Prepares the uterus to receive a fertilized egg. Adrenal cortex Epinephrine Stimulates “fight or flight” response. Adrenal medulla Glucocorticoids Part of stress response, increase blood glucose levels and decrease immune response. Aldosterone Regulates sodium content in the blood. Testosterone (in both sexes) Adult body form (greater muscle mass), libido. Pineal gland Melatonin Sleep cycles, reproductive cycles in many mammals.